1. Field of the Invention
The present invention relates to a dielectric resonator, a filter, a duplexer and a communication apparatus for use in the bands of microwaves, millimeter waves and so on.
2. Description of the Related Art
Recently, high-capacity and high-speed communication systems have been required to cope with a rapid increase in needs of mobile communication systems and a quick shift to the multimedia society. In response to an increased amount of information to be communicated, the frequency band for use in communications is going to be enlarged from the microwave band to the millimeter-wave band. In the millimeter-wave band, a conventional TE01.delta.-mode dielectric resonator formed of a columnar dielectric can also be used as in the microwave band. The resonance frequency of the TE01.delta.-mode dielectric resonator is determined depending on the externals dimensions of the columnar dielectric, and strict machining accuracy has been required to achieve the desired resonance frequency. Because the outer circumference and height of the columnar dielectric are set by grinding, it has been difficult to precisely set strict dimensions with respect to the resonance frequency in the millimeter-wave band where stricter machining accuracy is required.
Also, when a dielectric filter is constructed by arranging a plurality of TE01.delta.-mode dielectric resonators in a metallic case with predetermined intervals between, the resonators have been required to be arranged with high position accuracy because the coupling between input/-output means such as a metallic loop and the dielectric resonator or the coupling between the dielectric filter and the dielectric resonator is determined depending on the distance between those components.
With a view of solving the above problems, the inventors have proposed in Japanese Patent Application No. 7-62625 a dielectric resonator superior in machining accuracy and a dielectric filter superior in position accuracy.
A basic construction of the dielectric filter according to the above Japanese Patent Application is shown in FIG. 6. FIG. 6 is an exploded perspective view of the dielectric filter according to the above Japanese Patent Application.
As shown in FIG. 6, a dielectric filter 101 is made up of a dielectric substrate 102 and a pair of upper and lower conductor cases 103, 104.
The dielectric substrate 102 is a substrate having a predetermined relative dielectric constant, and has an electrode 102a formed all over one principal plane thereof except two circular openings 102c each having a predetermined diameter and an electrode 102b formed all over the other principal plane thereof except two circular openings 102d each having a predetermined diameter. The openings 102c, 102d each formed two in the respective principal planes are positioned to face each other.
The upper conductor case 103 is made of a metal and has a box-like shape with a lower surface being open. Also, the upper conductor case 103 is arranged while leaving a spacing from the dielectric substrate 102 near the openings 102c in the electrode 102a.
The lower conductor case 104 is made of a dielectric and has a box-like shape with an upper surface being open and flanges laterally projecting at the bottom. Also, a shield conductor 106 is formed on an inner peripheral surface of the lower conductor case 104, and input/output electrodes 105a, 105b are formed in positions facing the two openings 102d in the electrode 102b, respectively, in such a manner as isolated from the shield conductor 106. The input/output electrodes 105a, 105b are led out respectively through holes 104a, 104b formed in a side surface of the lower conductor case 104.
Further, a pair of spacers 107 are disposed in the lower conductor case 104 to keep a predetermined spacing between an inner bottom surface of the lower conductor case 104, on which the shield conductor 106 is formed, and the dielectric substrate 102. The spacers 107 are made of a dielectric material having a so low dielectric constant as not to disturb the electromagnetic field in the upper and lower conductor cases 103, 104.
In the dielectric filter having such a structure, electromagnetic energy is confined in the dielectric substrate 102 near its portions each sandwiched between the two opposing openings 102c, 102d in the electrodes 102a, 102b, causing those portions to serve as two TE010 mode resonators. As a result, a dielectric filter having resonators in two stages is obtained.
With the above-stated construction, the resonance areas are defined by the size of the openings in the electrodes and the openings can be formed by etching or ether like technique in the manufacture process. Hence a dielectric filter can be manufactured in which dimensional accuracy of resonators and position accuracy between the resonators with respect to the resonance frequency are very precisely reproduced.
In the above dielectric filter 101, however, since electromagnetic energy is confined at a high degree, the coupling between the resonators adjacent to each other has been inevitably weak. Accordingly, when the dielectric filter 101 is manufactured in practice, a narrow-band filtering characteristic has been necessarily resulted due to the weak coupling between the resonators adjacent to each other.
More specifically, when the dielectric filter 101 having a central frequency of 25 GHz was manufactured on condition that a dielectric ceramic substrate being 10 mm.times.6 mm square and 1 mm thick and having a relative dielectric constant of 24 was used as the dielectric substrate 102, the electrodes 102a, 102b were made of gold, the diameter of the openings 102c, 102d was 3.5 mm, the distance (gap) between the two openings 102c adjacent to each other or the distance (gap) between the two openings 102d adjacent to each other was 0.1 mm, the distance from the inner ceiling surface of the upper conductor case 103 to the upper surface of the dielectric substrate 102 was 1 mm, and the distance from the lower surface of the dielectric substrate 102 to the inner bottom surface of the lower conductor case 104 was 1 mm, the coupling coefficient was less than 1.5% and a resulting band-pass filter had a narrow band with a relative pass band width of approximately 300 MHz.
To make wider the band width of such a band-pass filter, it is conceivable to increase the coupling coefficient by reducing the distance between the resonators (the distance, i.e., gap, between the two openings 102c adjacent to each other or the distance between the two openings 102d adjacent to each other). There is however a limit in reducing the distance (gap) between the resonators. In practice, a limit of the distance (gap) between the resonators is 0.01 mm. It has been proved that, even in reducing the gap to such a limit value, the coupling coefficient is approximately 2% and the relative pass band width is approximately 400 MHz at maximum.
Furthermore, reducing the distance between the resonators means is equivalent to making smaller the distance between the two openings 102c adjacent to each other or the distance between the two openings 102d adjacent to each other, and hence has accompanied another problem of making it more difficult to effect patterning of the electrode 102a or 102d.
In addition, because of weak external coupling between the input/output electrodes 105a, 105b and the resonators, it has been necessary to optimally arrange the position relationship between the two openings 102d, which are formed in the electrode 102b on the other principal plane of the dielectric substrate 102, and the dielectric strips 105a, 105b for the sake of providing the required external coupling. There has been a difficulty in design of the above optimum arrangement.